Most electromagnetic induction (EMI) metal detectors use a loop antenna to create a magnetic field in the vicinity of a metal target for the purposes of detection and identification. One of the most important functions of a magnetic field antenna is to project a strong magnetic field at the site of the target. One of the consequences of the loop antenna’s complex spatial field strength is the fact that a metal target is excited with a complex magnetic field. When a buried target of unknown depth is scanned with an EMI sensor, the spatial distribution of the excitation magnet field at the target is not known. Some target identification algorithms assume that the target is excited with a uniform magnetic field, if the magnet field is in fact complex, the target’s time or frequency response to the field is not well characterized. This may tend to complicate or confuse a target identification algorithm. In addition, with the target at the center of the loop, the loop magnetic field antenna only measures the vertical component of the target’s decay response.

The Johns Hopkins University Applied Physics Laboratory has developed and is in the patent process for a method for identifying a buried metal object using a three-dimensional Steerable magnetic field (3DSMF) system. The method comprises of generating a magnetic field vector (MFV) at the first MFV position above the buried metal object; measuring a time decay response at the first MFV position: generating a MFV at a next MFV position; repeating the above steps until a complete 360-degree measurement of time decay responses of the buried metal object is completed; processing all measured time decay responses with a target search algorithm to determine a magnetic polarizability tensor of the buried metal object; and identifying the buried metal object by matching the magnetic polarizability tensor of the buried metal object to a known magnetic polarizability tensor of an object.